Improvement in industrial electric resistance heater

ABSTRACT

An electric resistance heater for industrial use with high heat emmisitivity per unit of length and low energy loss has coaxially-extending non-magnetic resistance units with a terminal end and an end opposite the terminal ends where the two conductors, otherwise electrically separate, are joined, thereby connecting them in series. At the terminal end the inner conductor projects through an enlargement of the outer conductor and has an exposed terminal portion of the same diameter as the enlarged terminal end of the outer conductor.

A United States Patent 1 Middough et a1.

[54] INDUSTRIAL ELECTRIC RESISTANCE HEA R [75] Inventors: William V.Middough, Shaker Heights, Ohio; Louis 11. Jaquay,

Bridgeville, Pa. [73] Assignee: Dravo Corporation, Pittsburgh, Pa. [22]Filed: Oct. 1, 1973 [21] Appl. No.: 402,092

Related US. Application Data [60] Division of Ser. Nos. 234,689, Jan.27, 1972, Pat. No. 3,764,718, Continuation ofSer. No. 29,150, April 16,

1 l970,abandoned.

[52] US. Cl 219/553, 13/25, 219/536, 338/237, 338/272 [51] Int. Cl. H05b3/10 [58]. Field of Search 13/16, 20, 22, 23, 25,

[56] References Cited UNITED STATES PATENTS 1,933,532 '10/1933 Nichols13/25 1,947,793 2/1934 O'Neil et a1. 338/235 1,951,753 3/1934 George eta1. 13/20 2,271,838 2/1942 Hanawalt et a1 13/20 2,355,343 8/1944 VonZeerleder et al. 13/20 X [11] 3,835,296 Sept. 10, 1974 2,623,080 12/1952Young 13/16 2,858,403 lO/1958 Butler, Jr... 338/229 2,971,039 2/1961Westeren... 13/25 3,147,331 9/1964 Brugger 13/23 3,281,517 10/1966Hemmer et al.... 13/31 3,571,476 3/1971 Anthony 13/20 FOREIGN PATENTS ORAPPLICATIONS 215,231 9/1941 Switzerland 13/25 450,959 7/1936 GreatBritain 219/553 915,619 7/1946 France.... 13/25 1,315,187 12/1962 France219/553 Primary Examiner-Velodymyr Y. Mayewsky Attorney, Agent, orFirm-Parmelee, Miller, Welsh & Kratz [5 7] ABSTRACT An electricresistance heater for industrial use with high heat emmisitivity perunit of length and low energy loss has coaxially-extending non-magneticresistance units with a terminal end and an end opposite the terminalends where the two conductors, otherwise electrically separate, arejoined, thereby connecting them in series. At the terminal end the innerconductor projects through an enlargement of the outer conductor and hasan exposed terminal portion of the same diameter as the enlargedterminal end of the outer conductor.

9 Claims, 17 Drawing Figures ass-5.296

' PATENTEDSEPI 01w sum 1 or 4 El i bu? PATENIEU SEP 1 0 1914 saw 3 OF HPAIENIEBSEP 1 01914 .JII

I INDUSTRIAL ELECTRIC RESISTANCE HEATER This is a division ofapplication Ser. No. 234,689 filed Jan. 27, 1972, now U.S. Pat. No.3,764,718, and which is a continuation of application Ser. No. 29,150,tiled Apr. .16, 1970, now abandoned.

This invention is for an electrical resitance heater, and moreespecially a resistance heater for use in certain industrialenvironments, particularly enclosed chambers, such as a molten metaldegassing chamber, where a large capacity heater is necessary, and whichis often subjected to especially destructive conditions.

Since the invention herein disclosed was developed especially for use inthe preheating of a vacuum metal degassing chamber, it will be describedin connection with such an apparatus, but without limitation to its usein other apparatus to which it is also applicable.

In the degassing of molten metal, such as steel, the molten metal isdrawn from a receptacle such as a ladle up a tubular leg and dischargedinto a refractory-lined degassing chamber which is connected to a pumpof some kind that maintains a vacuum therein. The reduced pressure inthe degassing chamber releases gases that are entrained in the metal.The degassed metal may then be returned to the receptacle from which itwas withdrawn or discharged into another vessel or into a continuouscasting apparatus.

Before the degassing chamber can be put into operation, it must bepreheated to bring the interior lining of the vessel up to asufficiently high temperature that the molten metal will not be chilledand solidify therein. Preheating the degassing chamber is sometimesaccomplished by burning has inside the chamber. However, electricresistance heaters are desirable, and the heater of the presentinvention is especially, but certainly not exclusively, for thispurpose.

One type of resistance heater heretofore designed for this purpose isthe straight through-type where a resistance element extends more orless diametrically through the interior of the degassing chamber withits terminals passing through the walls of the vessel and with electricconnections at the opposed protruding ends of the element. This requiresthat therebe two opposite ports in the wall of the vessel through whichopposite ends of the heater extend and both ports must be sealed againstair leakage when the chamber is under vacuum. The seals must beadequately protected by the refractory lining of the vessel. In suchconstructions the resistor is usually left in place when the apparatusis op erating, exposing the resistor both to destructive gases and tobeing splashed by not metal. When such a heater is energized withalternating current, there may be substantial current losses due to theinduction of eddy currents in the steel vessel or other metal objectsadjacent the heater.

A second type of heater which requires only a single port in the vesselwall has a resistor of U shape or hair pin" loop, but it requires acomplex seal and electric terminals and the resistor may become fouledand short-circuited by a build-upof metal, slag, or other foreignsubstances across the space between the two parallel legs of theresistor.

The present invention has for its principal objects to provide aresistance heater having a unique arrangement of resistor elements toreduce upkeep costs, avoid any external magnetic field, and in which theelectric terminals are at the same end of the resistor, so that only oneport is required in the furnace wall where the heater passes through thefurnace wall of an enclosure. However it is not subject toshort-circuiting by fouling of the resistor element. A further object isto provide with such a resistor a unique arrangement for connecting thesame to a source of electric power. A still further object is to providea resistor which may be withdrawn into a protected environment when itis not in use.

BRIEF DESCRIPTION OF THE INVENTION In general, the resistor of thepresent invention in its simplest form comprises an outer tubularelement, and positioned within it is a coaxially-extending innerelement, these elements being spaced from one another throughout theirlength, but they are connected at the inner end of the resistor by aconducting closure member which substantially excludes furnace gasesfrom entering the interior of the heater. The other or outer end of theouter tube has a terminal portion to which a current input connector maybe clamped. The outer end of the inner element also has a terminalportion that extends through and projects beyond the terminal portion ofthe outer member for attachment to the second current supply line. In adesirable installation, as for degassing, there is a tubular housing orshroud that is secured to the top of the degassing chamber, and theresistor may be lowered from a raised position where it is enclosed inthe housing to an operating position where it extends down into thedegassing chamber to radiate heat to the walls of said chamber. Spacedpairs of opposed clamping elements are so located on the housing thatwhen the heater is lowered into operating position, one pair of clampswill engage the upper end or terminal portion of the outer sleeveelement, and one pair of clamps will engage the protruding end of theinner resistor element to supply electric current thereto so that thetwo elements of the resistor are connected in series.

In most cases the outer tubular element and the inner element are bothheat-generating resistors, but in some cases the outer member may beoperated primarily as a conductor and as a radiant element that receivesthe heat generated by the inner resistor element and reradiates it tothe exterior. In other cases there may be two or more resistors in theouter member.

My invention may be more fully understood by reference to theaccompanying drawings, in which:

FIGS. 1A and 1B are complementary longitudinal sections through theresistor apart from the apparatus with which it is used, 1A being theouter end portion and B being the inner end portion;

FIG. 2 is a transverse section in the plane of line II-II of FIG. 1, buton a larger scale, included in FIGS. 3 and 4;

FIG. 3 is a transverse section in III of FIG. 1;

FIG. 4 is a transverse section in the plane of line IVIV of FIG. 1;

FIG. 5 is a side elevation of a molten metal degassing apparatus withthe electrode and the accompanying apparatus mounted on the degassingVessel;

FIG. 6 is a vertical longitudinal section showing the clamps for makingelectrical contact with the resistor elements when the resistor is inoperating position;

the plane of line III- FIG. 7 is an enlarged fragmentary view of theapparatus shown in FIG. 6, but rotated 90 from FIG. 6, the view being avertical section so that the operating linkage is not seen;

FIG. 8 is a transverse fragmentary section in the plane of lineVIII-VIII of FIG. 7, but showing the clamp-operating linkage;

FIG.9 is a side elevation of the linkage shown in FIG. 8, the view beingan elevation of the part of the structure shown in FIG. 7;

FIG. 10 is a vertical fragmentary section of the shroud on alarger'section than FIG. 6, showing the lifting and lowering connectionat. the top of the heater;

FIG. 11 is a transverse horizontal section in the plane of line XIXI ofFIG. 10;

FIG. 12 is an enlarged detail view in horizontal section of a singleconnector clamp and its mounting, it being an enlargement;

FIG. 13 is a more or less schematic modification of the heater shown inFIGS. 1A and 1B, the view being partly in elevation and partly inhorizontal section;

FIG. 14 is a transverse section on line XIV-XIV FIG. 13;

FIG. 15 is a view' similar to FIG. 13 of another modification for usewith a three-phase alternating current; and

FIG. 16 is a transverse section in the-plane of line XVI-XVI Of FIG. 15.

As previously indicated, resistance heaters as herein contemplated maybe designed to operate at quite high temperatures, even approximatingthe melting point of steel. For industries, such as steel degassing,where production is reckoned in tonnage, they must be large. In thespecific example hereinafter described, the overall length of the heateris of the order of around twentythree feet, but it may be longer orshorter, depending on the use to which it is to be put. To develop suchhigh temperatures in resistors of this size, very-heavy currents areused and substantial current losses can occur in some environments ifthe heater in operation sets up an external magnetic field. v

A form of the resistance heater itself is shown apart from the apparatusor equipment with which it is used in FIGS. 1A and 18. Because of itslength, and in order to show the construction, it has been necessary toshow the outer or upper end portion of the resistor in FIG. 1A and theopposite end portion in FIG. 18. For economy of manufacture and repairor replacement of parts, very long heaters may be comprised oflongitudinal sections which are coupled by separable couplings.

The heater as shown in FIGS. 1A and 1B is comprised of two elements orassemblies, an outer tubular assembly 2, an inner assembly 3, and thereis a combined closure and connector element 4 at the inner end unitingboth members.

The outer tubular assembly 2 has a terminal element 5 at the outer end,a tubular resistance element 6 having one end screwed in the terminal 5at 7. While the tubular element 6 could be an integral tubing for thefull length of the resistor, it preferably extends to a coupling sleeve8, and a second similar tubular resistance element 9, also screwed intothe coupling 8, extends from said coupling with its opposite end screwedinto the terminal closure element 4. As seen in the drawing, the closureelement 4 is of generally cup-like section with an internally-threadedsleeve portion 10 into which the member 9 is screwed.

The inner assembly 3 of the resistor comprises an outer terminal portion11, preferably of a diameter about the same as the terminal portion ofthe outer member, enabling similar connector clamps to be used with bothterminals; It has a conductor portion 12 of the substantial or solidsection concentrically positioned in and spaced from terminal portion 5of the outer member. As here shown it is made in two sections, the outerportion of which is integral with terminal 11, and the inner portion 12ais joined to the outer section in tight abutting relation thereto by athreaded internal coupling 13. The reduced inner end 12b of section 12ais connected at 14 to the inner resistor element 15 by aninternally-threaded coupling 16. The resistor is here shown as a tubularelement, but may be a rod. It is preferably made in two or more sectionsjoined together in abutting relation by an internal threaded coupling,as indicated at 16. The innermost end of the resistor 15 is firmlyconnected to the interior end of the cup-like end member 4 by a solidscrew plug coupling 17 in a central bor in the member 4, which is alsoscrewed into the end portion 15 of the resistor. This screw coupling 17holds the inner element centered in the outer one with the two spacedfrom each other throughout their length. The two assemblies 2 and 3 arethus spaced from and insulated from each other except for the endconnection 4 which unites them electrically and mechanically.

At the cooler terminal end of the heater a composite insulating bushingstructure 18 surrounding the portion 12 of the inner element and securedin the outer end of the outer terminal member 5 keeps the two resistorelements in concentric spaced relation. Because of the small scale ofthe drawing the structure of this bushing arrangement is only indicatedin FIG. 1A but is shown in greater detail in FIG. 10 and willhereinafter be more fully described. The bushing 18 also encloses thespace inside the heater against free flow of air into it. There areshown radial spacing pins 19 arranged in groups of three at intervalsalong the length of the heater, these also serving to keep the inner andouter resistor elements in concentric spaced relation. In most casesthey may be unnecessary, but where they are used they are located wheretheir outer ends pass through the resistor element and bear against theinner surface of a coupling, as at 7 or 8. Arranged in this way they arelocated for convenient assembly or replacement.

The resistor elements and terminal portions and the plug 4 and externaland internal couplings may be formed of carbon, silicon carbide, or ametal or metal alloy of a type commonly used in resistance heaters orcombinations thereof. The terminal 11 of the inner element is here shownwith an end socket in which is secured insulation'20 around a connector20a into which is screwed a lifting eye 21 for use where the resistor issuspended in a vertical position, as hereinafter described.

The heater, like a hair-pin type of .heater, has both terminals at oneend, but it is of advantage over a hairpin type in that with oneresistor leg is centered within the other and the space between the twolegs of the resistor is enclosed so that metal or slag can never buildup and bridge across the two resistor elements of this invention as itmay and does do with a hair-pin loop type of resistor. In operation,substantially all of the heat generated by the inner resistor element isradiated to and through the outer one, which also radiates heatgenerated within itself, providing a heaterwith a relatively large heatradiating surface which is also 'a resistance heater with a smallerdiameter internal resistor to increase or boost the overall temperature.As above pointed out, the coaxial arrangement of the two resistorseliminates any external magnetic field, thereby re ducing energy loss toinduced fields in the surrounding structure. With the inner resistorsubstantially protected from corrosive atmospheres to which the heateris exposed, or from the free circulation of air about it, it may beoperated at higher temperatures or for longer periods of time or boththan a resistor freely exposed to the surrounding atmosphere. The outerresistor of course has no similar protection, but it is cheaper toreplace one resistor from time to time than two of them, as is the casewith a hair-pin type of resistor heater or two conventional resistors.Also the outer resistor of the present invention may be of a moreexpensive and corrosive or oxidation-resistant material than the innerone, or may have a protective surface composition thereover that has asoftoning point higher than the operating temperature of the resistorand does not react with the resistor.

One apparatus for which the heater is especially, though not exclusivelyuseful, is one in which it is used in a vertically elongated generallycylindrical enclosure, such as a hot metal degasser. In a degasser theheater is used to preheat the interior of the enclosure into whichmolten metal is drawn from a ladle by vacuum, one such apparatus beingshown in FIG. 5.

Referring to FIG. 5, 22 designates a typical vacuum degassing vesselhaving one or more depending legs 23. Where there is a single leg,molten metal is alternately sucked up from a ladle into the vessel 22,retained in the vessel for degassing, and then discharged into theladle. More frequently the vessel has two depending legs 23, one behindthe other as viewed in FIG. 5, and molten metal is sucked up one leg anddischarged from the other, as is well understood in the .art. There is aduct 24 leading from the upper portion of vessel 22 through which gasesare drawn to maintaina vacuum in the degassing chamber by means of apump of some type (not shown). In a typical installation,and forpurposes of illustration, the degassing chamber may be of a height ofaround 22 feet with a diameter of perhaps 9 or 10 feet, while, as abovenoted, the heater has an overall length of around 2l to 22 feet.

The heating apparatus of the present invention is mounted on top of thedegassing chamber and is designated generally bythe numeral 25 in FIGS.5 and 6. The top of the degassing chamber has a port 26 therethroughabove which is a flanged fitting 27 to which a housing 28 is fixed. Thishousing as here shown has spaced walls so that it may be cooled, eitherwith water or air, and is of an upwardly-increasing flat-sided elipticalsection. There is a tubular extension 28a bolted or sealed to the topthereof. At the top of the tubular extension there is provided aninternally-offset gas-tight casing 29. There is a sheave 30 inside thiscasing, and a second idler sheave 31 in the casing offset to one side ofthe sheave 30, and a motor-driven winch 32 is contained within anextension at the top of the casing 29. It is driven-by a motor andreducing gear mounted on the outside of the casing 29, but which is notshown in the drawings. A cable 33 from the winch 32 is reeved under thesheave 31 and over the sheave and passes centrally down through theshroud and extension 28, and is attached to the lifting eye 21 at theterminal 11 of the resistor for moving the entire'heater unit up anddown in the shroud as hereinafter more fully explained.

For uniting the inner and outer assemblies and allowing for relativethermal expansion and contraction, there is an eleptical flange-plate 35at the end of terminal 11 of the inner member, but which is insulatedtherefrom by insulating sheet 35'. The connector 20a and the shank ofthe screw-eye 21 pass up through the center of the flange plate. Thecomposite bushing structure 18 at the outer end of the outer assemblycomprises applate 37 with a center sleeve portion screwed into the outerend of terminal 5 and with an insulator ring 37a therein between thesleeve and portion 12 of the inner assembly. There are headed tie'rods38 passing upwardly through the plate 37 with insulators 39 separatingthem from the plate 37. These rods pass through plate 37 and their upperends pass through and are adjustably secured in a plate 40. This platerests on a Belleville spring assembly 41 that is confined against afixed abutment 42 on the shank of the lifting eye below plate 40.

The plate 35 carries a short tubular extension 35a which, as seen inFIG. 10, alwaysprojects up into the tubular housing or shroud, even whenthe heater is in its lowermost position, as hereinafter described toguide the heater, and it is raised or lowered inside the shroud. Thisextension 35a has peripherally-spaced pins- 35b on the exterior thereofwhich move in guideways on the interior of the shroud to hold the heaterfrom twisting, or from becoming snagged when the heater is lowered toits fullest extent. The shroud has double walls providing a spacebetween them for circulation of cooling water.

The housing 28 contains the parts through which electrical connection ismade with the terminal of the resistor when the heater is in itslowermost or operating position. At this time the major portion of theheater projects to its fullest extent down into the degassing vesselwith its lower end spaced only a few feet at most from the bottom of thedegassing chamber. When the heater is thus lowered, the resistorterminals will be inside the housing 28, but the guide section 35b willstill be inside the shroud.

Electrical connection is made to the terminal 11 of the inner resistorelement by a pair of opposed clamps and 50a respectively and to theterminal 5 of the outer resistor element by a similar pair of opposedclamps 51 and 510, these being directly under the respective clamps 50and 50a. The larger diameter of the terminal portion 5 of the outerassembly with its corresponding increase in area and the enlargedportion 11 on the conductor 12-120 of the inner heating element provideenlarged areas for contact with the electrical connector clamps tothereby lower the contact resistance between the clamps and theconductors leading to the resistance heater elements themselves. Also itenables each pair of connector clamps is the same and each individualclamp is the same, so that one clamp of each pair and one pair will bedescribed, to apply to all. The clamps 50 and 50a for example may beformed of copper or graphite. Each has a curved inner face to conform tothe surface of the resistor terminal which is contacts. Each contactelement or shoe is at the inner end of a reciprocable conductor stem 52.This stem is slidably guided in insulating bushings 53 with a supportingsleeve 54 carried on a water-cooled supporting assembly 55 welded to andpassing through the housing 28. On the stem 52 beyond the end of thesleeve 54 there is a fixed disk or'plate 56. A metal bellows 57 abutsagainst and is joined to the inner face of this plate through a layer ofinsulation 58. As thus assembled, each Connector can move toward andaway from the terminal of the resistor which it is adapted to contact,and each one is insulated from the housing 8, but the bellows providesaseal to prevent leakage of air from outside the bellows into thesupporting structure 28,

. vessel 22 and all passages opening into it.

In FIGS. 8 and 12 the contact shoe 50 is retracted from and out ofcontact with the terminal 11, and at the same time all the other contactshoes 51 and 51a are retracted. To operate the contact shoes 50-50a topress tightly against the terminal 11, there is a fluid pressurecylinder 60, preferably designated to be operated by air pressure. Oneend of this cylinder is pivotally connected to one end of a lever 61that pivots about a fixed post 53, and which has its other end pivotallyconnected at 63 to the outer bifurcated end of the stem 52 through ashort link 61a pivoted to 52 at 59. The cylinder has a piston and pistonrod 64 connected to a similar lever 65 that pivots about post 66 and issimilarly connected to the stem 52 of the shoe 50a, the arrangementbeing such that when air under pressure is applied to cylinder 60, bothlevers will be rocked to move their respective shoe-supporting stems 52toward each other to clamp terminal 11 between them. To equalize theirrespective movements, lever 61 has a bushing thereon from which extendsa lever'68 and there is a similar lever 69 connected with lever 65.Links 70 connect the levers 68 and 69 to opposite ends of a horizontallypivoting equalizing bar 71 so that each shoe moves the same distance andthe pressure of each against the terminal 11 is equalized.

As above stated, the contact shoes 51 and 51a are arranged and operatedin the same way. Since each shoe is insulated from the housing 28, theupper contact shoes are insulated from the lower ones, so that thecurrent path, must be from one heater resistor terminal, through thefull length of one resistor across plug 4 and back through the fulllength of the other resistor.

-When' the degassing apparatus is operating, the heater is not required,and it is drawn by a winch 32 and cable 33 up into the tubular housingor shroud 28a, at which time of course the clamping electrodes areretracted. At start-up, however, when the degassing vessel is cold,theheater is lowered by the winch to the full length of the cable 33, atwhich time the terminal 11' of' the inner resistor member willbepositioned between the upper clamping electrodes 50 and 50a and theterminal of the outer resistor will be positioned between the lowerclamping electrodes 51-5la. .The several fluid pressure cylinders areenergized to move and desirable to have three resistor'elements, whichcould be concentric, or two parallel resistors may be positioned insidea third element or outer tube which may be a resistor, or simply acurrent-carrying element and re-radiator of heat generated in the tworesistors inside it. Such an arrangement is shown in FIGS. 13 and 14. Inthese figures 70 designated an outer tube having a terminal 71 at oneend. Extending lengthwise through the tube 70 are two parallel resistors72 and 73 which may be tubes or rods. The tube 70 and resistors 72 and73 may be made in section with couplings as shown in FIGS. 1A and 1B.

In FIGS. 13 and 14 the two resistors 72 and 73 have their inner endssecured in a conducting plug or body 74 that is screwed into orotherwise fixed in the end of tube 71 and in addition to forming acurrent conducting path from tube 70 to resistors 72 and 73, forms aclosure for the lower or inner end of the tube 70..At the opposite orouter end of the tube 70 the resistors 72 and 73 pass through a ceramicinsulating bushing 75 that serves to support them in parallel relationand insulate them from terminal 71. The outer ends of the re sistors 72and 73 are connected into a conducting terminal 76.

In this arrangement current may flow between terminals 71 and 76 througha path comprising tube 70, plug 74 and resistors 72 and 73. If desired,tube or element 70 may be a resistor, or it may be primarily a currentconducting enclosure for resistor elements 72 and 73. Whether 70 is aresistor or primarily a conductor, heat generated in resistors 72 and 73is radiated to element 70 and re-radiated from element 70 to thesurrounding environment.

FIGS. 15 and 16 show another modification in which the outer element issimply an enclosure for three resistor elements which generate the heatthat is radiated from the resistors. In these figures 80 is a tubularouter member, which may here be non-conducting, and within it are threeparallel separated resistors 81, 82 and 83. They are connectedelectrically at the lower or inner end by a'conducting closure plug 84that excludes air and furnace gases from freely circulating inside thetube 80. The upper ends of the three resistors pass through aninsulating closure at the top or outer end of the tube 80. This closurekeeps the upper ends of the resistors spaced from each other andprevents the free circulation of air or gases into the interior of thetube.

Each resistor has a terminal portion 81a, 82a and 83a respectively atits outer end.

This arrangement is especially suitable for use with a three-phasealternating current where the current paths may be from 81 through 84and 83, or 81 through 84 and 82, or through 82 through 84 and 83.

To the extent applicable, any arrangement or description shown in anyofthe several heaters may be utilized in any of the others.

I claim:

1. An electric resistance heater unit providing a large heat-generatingcapacity in respect to its diameter comprising coaxially-extendingresistance heating elements, one positioned within the other, the innerone being mechanically supported by the outer one and being electricallyconnected to the outer one only at oneend of the "heater unit, the outerheating element having a cylindrical terminal portion extending axiallybeyond both heating elements at the other end of the heater unit, theinner heating element having a terminal portion at said other end thatextends axially through the terminal portion of the outer heatingelement and which projects beyond the end of the outer heating element,but which is electrically insulated from the outer heating element, saidheating elements being detachably secured to their respective terminalportions and comprising of sections detachably secured together inend-to-end relation,

said sections comprising the outer heating element being joined byexternal sleeve couplings and the sections of the inner heating elementbeing joined by internal axially-extending connectors,

and electric insulator means inside the outer heating element atintervals therealong supporting the innter heating elementconcentrically within the outer one.

2. An electrical resistance heating unit as defined in claim 1 whereinthe inner heating element is mechanically supported by and electricallyconnected to the other one at said first end through an electricallyconductive gas-excluding plug.

3. An electric resistance heater unit as defined in claim 1 wherein theterminal portion of the inner element which projects beyond the outerelement has an enlarged cylindrical portion of a diameter substantiallythe same as the diameter of the terminal portion of the outer elementwhereby terminal clamps of like shape and design and range of movementcan be used to supply current to the respective terminals of the twoelements.

4. An electric resistance heater unit as defined in 30 first platemember secured to the free end of the terminal of the outer element andwhich has an insulating bushing therein through which the terminal ofthe inner element is slidably guided, a second plate member connectedwith the first through tie rods, and a compression spring operativelyinterposed between said second plate member and the free end of theterminal of the inner element.

7. An electric resistance heater unit as defined in claim 1 whereinthere is a housing surrounding that end of the heater from which theconducting terminal of the inner resistor protrudes, twoaxially-separated terminal clamping arrangements carried by the housinghaving contact shoes movable radially toward and away from the heater,one such terminal clamping arrangement being positioned to clamp theprotruding end of the terminal of the inner element, and the other beingat a position to contact the end portion of the terminal of the outerelement, said terminal clamping arrangements being electricallyinsulated from each other.

8. An electric resistance heater unit as defined in claim 1 whereinthere is a housing surrounding the terminal portion of the two elements,a pair of opposed upper contact shoes supported by the housing formovement into and out of clamping engagement with the outer terminalportion of the inner resistor, a second pair of opposed contact shoesbelow the upper ones, also supported on the housing for movement intoand out of clamping engagement with the terminal portion of the outerelement, and fluid pressure means for operating said shoes into and outof clamping engagement with the respective terminals, the outer terminalportion of the inner resistor and the exterior of the terminal portionof the outer resistor being cylinders of substantially the samediameter.

9. An electric resistance heater unit as defined in claim 8 in whichthere is a shroud carried on and fixed to said housing, and means forraising the heater unit into said shroud when the shoes are retractedfrom clamping engagement with said terminals.

1. An electric resistance heater unit providing a large heatgeneratingcapacity in respect to its diameter comprising coaxially-extendingresistance heating elements, one positioned within the other, the innerone being mechanically supported by the outer one and being electricallyconnected to the outer one only at one end of the heater unit, the outerheating element having a cylindrical terminal portion extending axiallybeyond both heating elements at the other end of the heater unit, theinner heating element having a terminal portion at said other end thatextends axially through the terminal portion of the outer heatingelement and which projects beyond the end of the outer heating element,but which is electrically insulated from the outer heating element, saidheating elements being detachably secured to their respective terminalportions and comprising of sections detachably secured together inend-to-end relation, said sections comprising the outer heating elementbeing joined by external sleeve couplings and the sections of the innerheating element being joined by internal axially-extending connectors,and electric insulator means inside the outer heating element atintervals therealong supporting the innter heating elementconcentrically within the outer one.
 2. An electrical resistance heatingunit as defined in claim 1 wherein the inner heating element ismechanically supported by and electrically connected to the other one atsaid first end through an electrically conductive gas-excluding plug. 3.An electric resistance heater unit as defined in claim 1 wherein theterminal portion of the inner element which projects beyond the outerelement has an enlarged cylindrical portion of a diameter substantiallythe same as the diameter of the terminal portion of the outer elementwhereby terminal clamps of like shape and design and range of movementcan be used to supply current to the respective terminals of the twoelements.
 4. An electric resistance heater unit as defined in claim 3wherein means are attached to said enlarged cylindrical terminal portionfor suspending the heater in a vertical position.
 5. An electricalresistance heater unit as defined in claim 1 having means mechanicallyconnecting the terminal portions of both elements arranged toresiliently compensate for unequal thermal expansion of the elements butto keep said terminal portions electrically separate.
 6. An electricresistance heater unit as defined in claim 4 wherein said connectingmeans comprises a first plate member secured to the free end of theterminal of the outer element and which has an insulating bushingtherein through which the terminal of the inner element is slidablyguided, a second plate member connected with the first through tie rods,and a compression spring operatively interposed between said secondplate member and the free end of the terminal of the inner element. 7.An electric resistance heater unit as defined in claim 1 wherein thereis a housing surrounding that end of the heater from which theconducting terminal of the inneR resistor protrudes, twoaxially-separated terminal clamping arrangements carried by the housinghaving contact shoes movable radially toward and away from the heater,one such terminal clamping arrangement being positioned to clamp theprotruding end of the terminal of the inner element, and the other beingat a position to contact the end portion of the terminal of the outerelement, said terminal clamping arrangements being electricallyinsulated from each other.
 8. An electric resistance heater unit asdefined in claim 1 wherein there is a housing surrounding the terminalportion of the two elements, a pair of opposed upper contact shoessupported by the housing for movement into and out of clampingengagement with the outer terminal portion of the inner resistor, asecond pair of opposed contact shoes below the upper ones, alsosupported on the housing for movement into and out of clampingengagement with the terminal portion of the outer element, and fluidpressure means for operating said shoes into and out of clampingengagement with the respective terminals, the outer terminal portion ofthe inner resistor and the exterior of the terminal portion of the outerresistor being cylinders of substantially the same diameter.
 9. Anelectric resistance heater unit as defined in claim 8 in which there isa shroud carried on and fixed to said housing, and means for raising theheater unit into said shroud when the shoes are retracted from clampingengagement with said terminals.